5.32 WWDG Guide

The example first clears the early wakeup interrupt flag and checks whether the last reset came from the WWDG, delays 1 second in advance to avoid download problems, then configures and starts the WWDG, and finally refreshes the counter in the main loop.

Data path: WWDG_ClrEWINTF clears the flag → RCC_FLAG_WWDGRSTF reset flag → WWDG_SetPrescalerDiv, WWDG_SetWValue, WWDG_Enable write the WWDG registers → the main loop WWDG_SetCnt refreshes the counter → LED3 toggles and LED1 lights up on a reset

1. Initialize the log, the indicators and the interrupt flag. main first calls log_init, clears the early wakeup interrupt flag, initializes LED1 (PA3) and LED3 (PA8) as push-pull outputs and turns them off, then enables the PWR clock.

    log_init();
    log_info("--- WWDG demo reset ---\n");
    WWDG_ClrEWINTF();
    LedInit(GPIOA, LED1 | LED3);
    LedOff(GPIOA, LED1 | LED3);
    /* Enable PWR Clock */
    RCC_EnableAPB1PeriphClk(RCC_APB1_PERIPHEN_PWR, ENABLE);

When porting to another board, only the LED1 and LED3 pin macros need to change; WWDG_ClrEWINTF must be called before enabling the WWDG to avoid a leftover flag from the previous run.

2. Check the reset source and display it. RCC_FLAG_WWDGRSTF is read: if this reset was caused by the WWDG, LED1 lights up, a message is printed and the reset flags are cleared, otherwise LED1 is turned off.

    /* Check if the system has resumed from WWDG reset */
    if (RCC_GetFlagStatus(RCC_FLAG_WWDGRSTF) != RESET)
    {
        /* WWDGRST flag set */
        /* Turn on LED1 */
        LedOn(GPIOA, LED1);
        log_info("reset by WWDG\n");
        /* Clear reset flags */
        RCC_ClearResetFlag();
    }
    else
    {
        /* WWDGRST flag is not set */
        /* Turn off LED1 */
        LedOff(GPIOA, LED1);
    }

When porting, keep the reset flag clear call, otherwise the next power-up still reads a stale reset source.

3. Delay 1 second before enabling. When the window value is very small the system may be in a frequent reset state, which prevents the program from being downloaded normally; the example delays 1 second here to avoid that.

    /*
      When the window value is very small, the system is in a frequent reset state,
      at this time, easy to cause the program can not download normally.
      Add a delay of 1 second here to avoid this phenomenon. Of course,
      it can also be downloaded without delay, directly pull the pin of BOOT0 high.
    */
    systick_delay_ms(1000);

When porting, the delay can be shortened to reduce the power-on wait, but the program must still be downloadable.

4. Configure and start the WWDG. After enabling the WWDG clock, the prescaler, window value and counter value are written in turn, and WWDG_Enable starts the counter while writing the counter value.

    /* WWDG configuration */
    /* Enable WWDG clock */
    RCC_EnableAPB1PeriphClk(RCC_APB1_PERIPHEN_WWDG, ENABLE);

    /* WWDG clock counter = 1/((PCLK1(120MHz)/4096)/8) = (~273 us)  */
    /* WWDG clock counter = 1/((PCLK1(100MHz)/4096)/8) = (~327 us)  */
    WWDG_SetPrescalerDiv(WWDG_PRESCALER_DIV8);

    /* Set Window value to 80; WWDG counter should be refreshed only when the counter
      is below 80 (and greater than 64) otherwise a reset will be generated */
    WWDG_SetWValue(80);

    /*
      When PCLK1 = 120MHz, in this case the refresh window is: ~273 us * (127-80) = 12.831 ms < refresh window < ~273 us * (127-63) = 17.472ms
      When PCLK1 = 100MHz, in this case the refresh window is: ~327 us * (127-80) = 15.401 ms < refresh window < ~327 us * (127-63) = 20.971ms
    */
    WWDG_Enable(127);

The refresh window is determined jointly by the prescaler, the window value and the current counter value, so changing any one of them requires checking the window range again.

5. Refresh the counter in the main loop. The loop toggles LED3, waits 16 ms and then refreshes the counter with WWDG_SetCnt(127).

    while (1)
    {
        /* Toggle LED3 */
        LedBlink(GPIOA, LED3);
        systick_delay_ms(16);
        /* Update WWDG counter */
        WWDG_SetCnt(127);
    }

The 16 ms delay falls inside the refresh window of the example; moving it outside the window triggers a WWDG reset.

Corresponding source files: src/main.c